Mid-rapidity anti-baryon to baryon ratios in pp collisions at $\sqrt{s}$ = 0.9, 2.76 and 7 TeV measured by ALICE

ALICE Collaboration

The ratios of yields of anti-baryons to baryons probes the mechanisms of baryon-number transport. Results for $\bar{\rm p}/{\rm p}$, $\bar{\rm \Lambda}/{\rm \Lambda}$, $\rm\bar{\Xi}$$^{+}/{\rm \Xi}^{-}$ and $\rm\bar{\Omega}$$^{+}/{\rm \Omega}^{-}$ in pp collisions at $\sqrt{s} = 0.9$, 2.76 and 7 TeV, measured with the ALICE detector at the LHC, are reported. Within the experimental uncertainties and ranges covered by our measurement, these ratios are independent of rapidity, transverse momentum and multiplicity for all measured energies. The results are compared to expectations from event generators, such as PYTHIA and HIJING-B, that are used to model the particle production in pp collisions. The energy dependence of $\bar{\rm p}/{\rm p}$, $\bar{\rm \Lambda}/{\rm \Lambda}$, $\rm\bar{\Xi}$$^{+}/{\rm \Xi^{-}}$ and $\rm\bar{\Omega}$$^{+}/{\rm \Omega^{-}}$, reaching values compatible with unity for $\sqrt{s} = 7$ TeV, complement the earlier $\bar{\rm p}/{\rm p}$ measurement of ALICE. These dependencies can be described by exchanges with the Regge-trajectory intercept of $\alpha_{\rm {J}} \approx 0.5$, which are suppressed with increasing rapidity interval ${\rm \Delta} y$. Any significant contribution of an exchange not suppressed at large ${\rm \Delta} y$ (reached at LHC energies) is disfavoured.

Figures

Figure 1

The $\rm{DCA_{xy}}$ distributions for pp at $\sqrt{s} = 2.76$ TeV for the lowest (left) and highest (right) $\pt$ bins. Protons (anti-protons) are shown with full (open) symbols.
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Figure 2

Cosine of pointing angle distributions for pp at $\sqrt{s} = 7$ TeV in the lowest (left) and highest (right) $\pt$ bins. $\rm \Lambda$ and $\overline{\rm \Lambda}$ are shown with full and open symbols, respectively.
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Figure 3

The invariant mass distributions for $\rm \Lambda$ (top), $\rm \Xi$ (middle) and $\rm \Omega$ (bottom) in pp at $\sqrt{s} = 7$~TeV. Areas considered as signal and background (green) or pure background (blue) are shown. The lines corresponds to a polynomial fit to the background areas.
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Figure 4

The ratio of the detection efficiency for $\overline{\rm p}$ (solid line) and $\rm K^-$ (dashed line) calculated from GEANT3 to the one calculated from FLUKA as a function of the hadron $\pt$.
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Figure 5

Absorption correction factors for protons, anti-protons and charged kaons.
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Figure 6

The $\rm{\overline{p}/p}$ ratio at $\sqrt{s} = 2.76$ TeV as a function of $\pt$ (top) and rapidity (bottom). The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the rapidity or $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 7

The $\rm{\overline{\Lambda}/\Lambda}$ ratio at $\sqrt{s} = 0.9$ TeV as a function of $\pt$ (top) and rapidity (bottom). The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the rapidity or $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 8

The $\rm{\overline{\Lambda}/\Lambda}$ ratio at $\sqrt{s} = 2.76$ TeV as a function of $\pt$ (top) and rapidity (bottom). The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the rapidity or $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 9

The $\rm{\overline{\Lambda}/\Lambda}$ ratio at $\sqrt{s} = 7$ TeV as a function of $\pt$ (left) and rapidity (right). The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the rapidity or $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 10

The $\XbarX$ ratio at $\sqrt{s} = 0.9$ TeV integrated over $|y|< $ 0.8 as a function of $\pt$. The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 11

The $\XbarX$ ratio at $\sqrt{s} = 2.76$ TeV integrated over $|y|< $ 0.8 as a function of $\pt$. The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 12

The $\XbarX$ ratio at $\sqrt{s} = 7$ TeV as a function of $\pt$ (top) and rapidity (bottom). The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the rapidity or $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 13

The $\ObarO$ ratio at $\sqrt{s} = 2.76$ TeV integrated over $|y|< $ 0.8 as a function of $\pt$. The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 14

The $\ObarO$ ratio at $\sqrt{s} = 7$ TeV integrated over $|y|< $ 0.8 as a function of $\pt$. The data points are compared with different Monte Carlo generators. The vertical bars (boxes) represent the statistical (systematic) uncertainty, while the horizontal bars represent the width of the $\pt$ bin. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 15

The mid-rapidity yields ratio integrated over $|y| < 0.5$ for $\pbarp$ and $|y| < 0.8$ for $\LbarL$, $\XbarX$ and $\ObarO$. Squares, triangles and circles are for the data from pp at $\sqrt{s} = 0.9$, $2.76$ and $7$ TeV, respectively. The strangeness content increases along the abscissa.
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Figure 16

Anti-baryon to baryon yields ratios as a function of beam rapidity for various baryons separately. The parametrisation with Eq. (1) (blue line) is shown. The red points show the ALICE measurements.
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Figure 17

$\pbarp$ and $\LbarL$ ratios as a function of rapidity at $\sqrt{s} = 0.9$ and $7$ TeV. The parametrisation with Eq. (1) (black line) is shown.
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Figure 18

Charged particle multiplicity distributions. The event samples are divided according to multiplicity bins used in $\pbarp$ ratio analysis. The insets show the probability for different bins.
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Figure 19

The $\pbarp$ ratio in pp collisions at $\sqrt{s} = 0.9$, $2.76$ and $7$ TeV as a function of the relative charged-particle pseudorapidity density. The data points are compared with prediction of PYTHIA (Perugia2011). The vertical bars (boxes) represent the statistical (systematic) uncertainty. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 20

The $\LbarL$ ratio in pp collisions $\sqrt{s} = 2.76$ and $7$ TeV as a function of the relative charged-particle pseudorapidity density. The data points are compared with prediction of PYTHIA (Perugia2011). The vertical bars (boxes) represent the statistical (systematic) uncertainty. Ratio of model to data is shown below using uncertainties added in quadrature.
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Figure 21

The $\XbarX$ ratio in pp collisions $\sqrt{s} = 7$ TeV as a function of the relative charged-particle pseudorapidity density. The data points are compared with prediction of PYTHIA (Perugia2011). The vertical bars (boxes) represent the statistical (systematic) uncertainty. Ratio of model to data is shown below using uncertainties added in quadrature.
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